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8. But the great improver of the thermometer was FAHREN
In the year 1714 he constructed thermometers, employing two fixed points in graduating them-one indicated by the melting-point of ice, and the other by the boilingpoint of water at the mean pressure of the atmosphere. The former point he called 32°, the latter 212o. Any improvements that have since been made in constructing thermometers, such as self-registration, are merely matters of convenience or detail. In so far as concerns the principle of construction, the thermometer may be regarded as having come perfect from the hands of Fahrenheit, since the instrument he invented can be reproduced by any one at pleasure, the indications being in all cases absolutely the same. invention soon bore excellent fruit. Small portable thermometers were constructed by Fahrenheit, which, being carried by travellers and medical men over every part of the world, furnished observations of the most valuable description. The comparative temperatures of different countries became known, and the exaggerated accounts of travellers with regard to excessive heat and cold prevailing in foreign countries, were reduced to their proper value. Thermometers were further turned to excellent account in the arts of brewing and horticulture, and to objects affecting public health, and thus substantial additions were made to our comforts and luxuries.
9. The Hygrometer. The expansion and contraction of vegetable and animal substances with the varying quantity of moisture in the atmosphere, would appear to have suggested the idea of the hygrometer, an instrument of great value in meteorology, as indicating the quantity of vapour in the air, and, inferentially, the changes of weather resulting from such variation. These substances were used as hygrometers by the earlier meteorologists, especially by De Saussure, whose ingenious and extensive researches, conducted with the simple hair hygrometer, entitle him to be considered the founder of this department of meteorology.
10. From the period of the invention of these instruments, the number of meteorological observers was greatly increased, and a large body of well-authenticated facts of the utmost value began to be collected. The climates of particular parts of the earth were inquired into, and compared together; and the science made great and rapid advances by the investigations undertaken by distinguished philosophers into the laws which regulate atmospheric phenomena.
IMPORTANT DISCOVERIES AND DISCUSSIONS.
11. The theory of the Trade-Winds was first propounded by John Hadley in the ‘Philosophical Transactions' for 1735; and it may be mentioned as a curious fact that it remained altogether unnoticed for half a century, when it was independently arrived at by Dalton, and published in his
, essays. 12. The publication of Dalton's • Meteorological Essays' in 1793 marks an epoch in meteorology. It was the first instance of the principles of philosophy being brought to bear on the explanation of the complex and varied phenomena of the atmosphere. The idea that vapour is an independent elastic fluid, and that all elastic fluids, whether alone or mixed, exist independently; the great motive forces of the atmosphere; the theory of winds, with their effect on the barometer, and their relations to temperature and rain; observations on the height of clouds, on thunder, and on meteors; and the relations of magnetism and the aurora borealis, —are some of the important questions discussed in these remarkable essays, with an acuteness, a fulness, and a breadth of view, which leave nothing to be desired.
13. One of the most interesting and fruitful subjects of inquiry that long engaged the attention of meteorologists was the origin of Dew. Pictet of Geneva, Le Roy of Montpellier, Six of Canterbury, and Patrick Wilson of Glasgow, contributed valuable observations and experiments which did much to elucidate the subject. Of these, the first place is unquestionably due to Patrick Wilson, whose Memoirs of Certain Great Frosts at Glasgow,' about 1780, show a fidelity of observation, and a skill in interrogating nature, which have rarely been surpassed. When it is considered that he missed the point of the argument in continuing to entertain the notion that the cold accompanying dew comes after instead of before its deposition, the genius he manifested in his experiments will appear all the more wonderful. It was reserved for Dr Wells to collect the different observations into a coherent whole, and account for all of them by the theory of dew he propounded--a theory so just and so complete that all succeeding observation and inquiry have only confirmed it. "The Theory of Dew' was published by him in 1814, and must always be regarded as one of the greatest contributions made to meteorology. The subject of radiation had been discussed by Halley, but his inquiries had reference only to solar radiation, or radiation from the sun to the earth. Radiation outwards from the earth towards space, or terrestrial radiation, was first taken account of by Lambert, in his Pyrometrie,' published in 1779. Prevost of Geneva had also published his Essay on Radiant Heat' in 1809, or five years before the appearance of Dr Wells's treatise. But as Dr Wells had not seen these essays, his discoveries regarding radiation were original and independent. He made this allimportant observation, or rather discovery, which neither Wilson nor Prevost had suspected—viz., that during those nights when dew is deposited, the temperature of bodies on the earth's surface is colder than that of the surrounding air. He also ingeniously applied the principles laid down in the essay to explain the manufacture of ice during night at Benares in India.
14. In 1823 Daniell published his Meteorological Essays and Observations,' in which he discussed in a masterly manner the hygrometry of the atmosphere, solar and terrestrial radiation, the barometric measurement of heights, the tradewinds, evaporation, and natural and artificial climates. While in all these departments he contributed largely to our knowledge, his attention was most successfully turned to the investigation of the hygrometry of the atmosphere. Though the practical advantages he anticipated as likely to flow from it have not been realised, yet this difficult, and, in some points, still obscure department of meteorology, is indebted to him more than to any other philosopher. The law of the diffusion of vapour through the air, its influence on the barometric pressure, and its relations to the other constituents of the atmosphere, are among the least satisfactorily determined questions in meteorology. Since this element is so important as an indicator of storms and other changes of the weather, it is to be hoped that it will soon be more thoroughly investigated.
15. A most important addition to our knowledge of the vapour of the atmosphere was made in 1862 by Professor Tyndall of London in his experiments on radiant heat, especially as regards the gases, by which it is shown that the vapour of water exerts extraordinary energy as a radiant and an absorbent of heat. As a consequence, the vapour dissolved in the air serves as a covering or protection to the earth, shielding it from the sun's heat by day, and from the chilling effects of its own radiation during night. It is to be expected that the discovery of the relations of atmospheric vapour to heat will soon be turned to account in explaining many questions in meteorological science.
16. Humboldt's treatise on ‘Isothermal Lines,' published in 1817, marks an important epoch in experimental meteorology. Dové has since continued the investigation, and in his splendid work On the Distribution of Heat on the Surface of the Globe,' has given charts of the world showing the mean temperature for each month and for the year, together with charts of abnormal temperature. It is scarcely possible to over-estimate the value of this work; for, though to some extent the lines are hypothetical, there can be no doubt that a close approximation to the march of temperature and its distribution over the globe through the year has been arrived at. The idea has been carried out with greater fulness of detail by the Government of the United States of America, in the beautiful and elaborate series of Charts of Temperature and Rainfall published in “The Army Meteorological Register' for 1855. In these charts, the temperature and rainfall during the different seasons, for every part of the United States, are laid down from accurate observations. Temperature Charts of the British Islands have also been published by the Scottish Meteorological Society, for the months of January, April, July, and October, these months being considered as representative of the four seasons. It is to be hoped, considering how much yet remains to be done in this direction, that Societies and Governments will undertake the preparation and publication of similar charts, which are of inestimable value, not merely as indicating the climates of different countries, but also as showing how the temperature of one country may, by the intervention of the winds, be affected by the temperature of surrounding countries. The important influence which the temperature of one country, differing materially from that of a neighbouring one at a particular season, has in causing unsettled and stormy weather in both at that time, is too obvious to be longer dwelt upon.
17. In connection with terrestrial temperature, the laborious investigations of Dové, Buys Ballot, Jelinek, Quetelet, Hansteen, Kupffer, Forbes, and Glaisher, in calculating the mean temperature of places for periods of five or of two days, or for each day of the year, deserve to be specially noticed. An examination of these mean daily temperatures brings out the interesting fact that, over extensive parts of the earth's surface, interruptions occur, at stated times in the year, in the regular rise and fall of temperature, thus pointing to widespread disturbing causes, the explanation of which will doubtless lead to a juster conception of the disturbing forces of the atmosphere.
18. On the 15th of June 1752, Benjamin Franklin, by the happily-conceived experiment of flying a kite, identified lightning and electricity, thus giving an interest and an impetus to electrical observations. The brilliant discoveries which have recently been made on the mutual relations of electricity, magnetism, heat, motion, and the other forces of matter,